The nonlinear optical characteristics, conductivity, and semiconductivity of an organic semiconductor compound have attracted attention in the field of optoelectronics, so that the development of various devices has been vigorously conducted.
In recent years, a field effect transistor (FET) element using an organic semiconductor compound for its semiconductor layer has been attracting attention. In view of this circumstance, an organic semiconductor compound is now considered to be suitable for the preparation of a flexible element using plastics as its substrate because the organic semiconductor compound exhibits flexible film property as compared to an inorganic material such as silicon.
Representative examples of such an organic semiconductor compound include phthalocyanine-based compounds and polyacenes. Characteristics such as nonlinear optical characteristics, conductivity, and semiconductivity necessary for preparing devices using those compounds as organic materials are known to largely depend on the crystallinity and orientation of the materials as well as their purities. However, it has been difficult to make many compounds such as the phthalocyanine-based compound and the polyacenes mentioned earlier, in which π conjugated systems are extended, highly pure partly because the compounds are insoluble in solvents and susceptible to oxidation in the atmosphere. Moreover, a large-scale apparatus has been necessary for film formation partly because vacuum evaporation should be performed in order to obtain a crystallized film having high orientation.
For instance, pentacene, a representative example of an organic semiconductor compound, can be formed into a film on a substrate by means of vacuum evacuation alone because pentacene has high crystallinity and is insoluble in a solvent.
Meanwhile, a FET is prepared more simply by forming a thin film using a solution of an organic semiconductor that is soluble in an organic solvent by means of a coating method such as a spin coating method. Examples of such a FET include one using a π-conjugated polymer for its semiconductor layer (see “Japanese Journal of Applied Physics”, Japan Society of Applied Physics, vol. 30, pp. 596-598, 1991). It is known that, in the case of a π-conjugated polymer, an arrangement state of molecular chains largely affect electric conductive characteristics. Similarly, it has been reported that a field effect mobility of a π-conjugated polymer field effect transistor is largely dependent on the arrangement state of the molecular chains in the semiconductor layer (see “Nature”, Nature Publishing Group, vol. 401, pp. 685-687, 1999).
However, the arrangement of the molecular chains of a π-conjugated polymer is performed during a period from solution coating to solution drying. Therefore, there is a possibility that the arrangement state of the molecular chains varies to a large extent owing to an environmental change or depending on coating method. In view of this, there has been reported a FET using a film obtained by: forming a thin film of a soluble precursor of pentacene through coating; and subjecting the thin film to heat treatment to transform the precursor into pentacene (see “Advanced Materials”, WILLEY-VCH Verlag GmbH, vol. 11, pp. 480-483, 1999). In this case, the transformation into pentacene necessitates high-temperature treatment, and an eliminated component having a large mass must be removed under reduced pressure.
As described above, a conventional FET element using an organic semiconductor compound has required a complicated step such as vacuum film formation, or involved a problem in that the element is easily affected by the environment.